The present invention relates to the field of spread spectrum radio receivers and, in particular, to a demodulator capable of operating in QPSK or FSK environments.
Many different digital modulation techniques exist, however, at bottom any of these techniques may be grouped into one of three categories: amplitude shift keying (ASK), frequency shift keying (FSK) and phase shift keying (PSK). ASK describes a technique wherein a carrier wave, having a certain frequency xe2x80x9cfcxe2x80x9d, is multiplied by a digital information signal, g(t). Mathematically, the modulated carrier signal, s(t), is then:
s(t)=g(t)sin(2xcfx80fct+xcfx86). 
ASK is a special case of amplitude modulation (AM) and has the property of translating the spectrum of the modulation g(t) to the carrier frequency without altering the bandwidth thereof. This process is variously known as mixing, up-conversion or down-conversion.
FSK describes the modulation of a carrier signal by using a different frequency to represent a logic 1 or 0. The resultant modulated carrier, s(t), may be regarded as the sum of two amplitude modulated signals of different frequency:
s(t)=f1(t)sin(2xcfx80fc1t+xcfx86)+f2(t)sin(2xcfx80fc2t+xcfx86). 
FSK is generally classified as wide-band or narrow-band depending upon whether or not the separation between the two carrier frequencies is larger or smaller, respectively, than the bandwidth of the spectrums of f1(t) and f2(t).
PSK describes a modulation technique wherein the phase of the carrier is altered to represent logic 1s and 0s. Mathematically:
s(t)=sin(2xcfx80fc+xcfx86(t)). 
PSK has several varieties, including binary PSK (BPSK), which has only two phases (0 and xcfx80), making BPSK a form of ASK with f(t) taking on values of xe2x88x921 or 1; and quadrature PSK (QPSK), which has four phases (0, xcfx80/2, xcfx80 and 3xcfx80/2). For a given bit rate, QPSK requires only half the bandwidth of PSK, making QPSK a popular modulation scheme.
The Institute of Electrical and Electronics Engineers (IEEE) is presently developing a standard to govern communications within Wireless Local Area Networks (WLANs) that relies on digital modulation techniques. That standard is referred to as IEEE 802.11 and it can be compared to the well-known IEEE 802.3 standard for communications within Ethernet wired LANs. The goal of the 802.11 standard is to provide a common operational model in order to resolve compatibility issues between different manufacturers of WLAN equipment. Accordingly, the IEEE 802.11 standards committee is preparing a version of a Media Access Controlxe2x80x94Physical Level (MAC-PHY) level specification.
Under the IEEE 802.11 standard, the fundamental access method of the 802.11 MAC is known as Carrier Sense Multiple Access with collision avoidance, or CSMA/CA. Familiar to those accustomed to wired Ethernet LANs, CSMA/CA works on a xe2x80x9clisten before talking schemexe2x80x9d. Any station wishing to transmit data within the LAN must first sense the radio channel to determine whether another station is currently transmitting. Only if the radio channel is not busy may the intended transmission may proceed. If a current transmission is detected, he CSMA/CA scheme uses randomized time gaps (called back-off intervals) to wait before xe2x80x9clisteningxe2x80x9d again to verify a clear channel. This process is repeated until the station is allowed to transmit. This type of multiple access scheme is meant to ensure judicious channel sharing while avoiding collisions.
The IEEE 802.11 PHY can be implemented in several ways, including diffused infrared (DFIR), direct sequence spread spectrum (DSSS) radio, and frequency hopped spread spectrum (FHSS) radio. Both of the spread spectrum radio techniques are used in the 2.4 GHz band because of wide availability in many countries and lower hardware costs in comparison to the higher microwave frequencies. The IEEE 802.11 standard supports DSSS for use with BPSK modulation at a 1 Mbps data rate, or QPSK modulation at a 2 Mbps data rate. The general band plan consists of five overlapping 26 MHz sub-bands centered at 2.412, 2.427, 2.442, 2.457, and 2.470 GHz. This scheme is used in an attempt to combat interference and selective fading. FHSS is supported with Gaussian FSK (GFSK) modulation and two hopping patterns with data rates of 1 Mbps and 2 Mbps. Under this scheme, the band is divided into 79 sub-bands of 1 MHz bandwidth each. Each sub-band is subject to a minimum rate of 2.5 hops/s using any of three possible hop patterns (22 hops in a given pattern). The minimum hop rate ensures that each packet sent could be transmitted in a single hop so that destroyed information could be recovered in another hop.
DSSS is a widely used form of spread spectrum and the DSSS process is performed by effectively multiplying an RF carrier and a pseudo-noise (PN) digital signal or code. First the PN code is modulated onto the information signal using one of several modulation techniques (e.g., BPSK, QPSK, etc). Then, a doubly balanced mixer is used to multiply the RF carrier and PN modulated information signal. This process causes the RF signal to be replaced with a very wide bandwidth signal with the spectral equivalent of a noise signal. The demodulation process (for the BPSK case) is then simply the mixing/multiplying of the same PN modulated carrier with the incoming RF signal. The output is a signal that is a maximum when the two signals exactly equal one another or are xe2x80x9ccorrelated.xe2x80x9d The correlated signal is then filtered and sent to a BPSK demodulator. The signals generated with this technique appear as noise in the frequency domain. The wide bandwidth provided by the PN code allows the signal power to drop below the ambient noise threshold without loss of information.
One feature of DSSS is that QPSK may be used to increase the data rate. This increase of a factor of two bits per symbol of transmitted information over BPSK causes an equivalent reduction in the available process gain. The process gain is reduced because for a given chip rate, the bandwidth (which sets the process gain) is halved due to the two-fold increase in information transfer. The result is that systems in a spectrally quiet environment benefit from the possible increase in data transfer rate.
FHSS relies on frequency diversity to combat interference. This is accomplished by multiple frequency, code selected, FSK. Basically, the incoming digital stream is shifted in frequency by an amount determined by a code that spreads the signal power over a wide bandwidth. In comparison to binary FSK, which has only two possible frequencies, FHSS may have 2*1020 or more frequencies.
An FHSS transmitter is a pseudo-noise PN code controlled frequency synthesizer. The instantaneous frequency output of the transmitter jumps from one value to another based on the pseudo-random input from the code generator. Varying the instantaneous frequency results in an output spectrum that is effectively spread over the range of frequencies generated. In such a scheme, the number of available discrete frequencies determines the bandwidth of the system. Hence, the process gain is directly dependent upon the number of available frequency choices for a given information rate. Another important factor in FHSS systems is the rate at which the hops occur. The minimum time required to change frequencies is dependent on the information bit rate, the amount of redundancy used, and the distance to the nearest interference source.
In one embodiment, a spread spectrum radio receiver configurable for use in both a quadrature phase shift keying (QPSK) and a frequency shift keying (FSK) environment is provided. The receiver may include a programmably selectable zero crossing detector unit for use when the receiver is configured for the FSK environment and/or programmable low pass filters having variable cut-off frequencies. A common local oscillator may be used regardless of whether the receiver is configured for use in the QPSK or FSK environment.
In a further embodiment, a spread spectrum radio receiver having a programmable option for selecting QPSK or FSK operation is provided. The programmable option may include the use of a summing operation when FSK operation is selected.